Walk-through metal detectors are conventionally used for screening passengers at airports, visitors and prisoners at courthouses and prisons, and various visitors and personnel at other high security risk areas, for detecting any gun, knife, contraband or other undesirably concealed metal object on an individual passing through the archway in which the detecting system operates.
In the past it has been considered sufficient to have such detection systems rechecked for accuracy only occasionally and seldom at more than one level of heigth of a typical person going through the archway. However, suppliers have recently started offering weapons which are harder to detect, including pistols made of low magnetic metals such as stainless steel, or consisting principally of non-magnetic components such as plastic or ceramic materials, or made very small. For example, there is now for sale a five shot stainless steel revolver using 22 caliber cartriges, which weighs only four ounces, and can easily be carried through a checkpoint in a hat worn on the head or else in the upper end of a sock worn on the leg. Such developments require frequent and accurate recalibration checks on the capability of metal detectors to pick up hard to detect metal objects, especially as they move at normal walking speed through a conventional checkpoint archway. To minimize expense, the rechecking system must not only be of simple construction but also must be operable by the non-technical personnel who regularly staff the checkpoint.